What techniques would allow a model to be trained to produce subjectively pleasant 'artistic' output?
You will need a large training set to be able to train your machine to produce subjectively pleasant 'artistic' output.

If you can't produce your own dataset with real people, you can try the following:


search for existing datasets on the internet
produce a new dataset using a spider bot that perform mining of artistic output on websites that have voting features
use crowdsourcing